In an image forming device, the heat roller type fixing device has been widely used as a fixing device which thermally fixes an unfixed image (a toner image) of object image information, which is formed and carried on a recording material (a transfer material sheet, an electrofax sheet, a sheet of electrostatic recording paper, an OHP sheet, a sheet of printing paper, a sheet of format paper and the like) by the transfer method or the direct method, on the recording material surface as a permanently fixed image in an image forming process means section of an electrophotographic process, an electrostatic recording process, a magnetic recording process and the like. In a fixing device of the heated roller type, it is general practice to use a heat source such as a halogen heater within the roller.
On the other hand, there have been widely proposed and carried out fixing devices of a type in which a resin belt or a metal belt having a small heat capacity is heated by using a ceramics heater as a heat source. That is, in fixing devices of this heating type, generally, a nip portion is formed by nipping a heat resistant belt (a fixing belt) between a ceramics heater as a heating body and a pressure roller as a pressurizing member, a recording material, on which an unfixed toner image to be fixed is formed and carried, is introduced between the fixing belt in the nip portion and the pressure roller, and the recording material is supported in a sandwiching manner and transported together with the belt, whereby in the nip portion, the heat from the ceramics heater is given to the recording material via the belt and the unfixed toner image is hot pressed and fixed on the surface of the recording material by this heat and the pressure load in the nip portion.
In this fixing device of the belt heating type, it is possible to make up a device of an on-demand type by using a small heat capacity member as a belt. That is, it is only necessary that the belt be heated to a prescribed fixing temperature by energizing the ceramics heater as a heat source only when the image formation is executed by the image forming apparatus. The fixing device of this type is advantageous in a short waiting time from the power-on operation of the image forming apparatus to a state in which the image formation can be executed (quick start capabilities) and a very small power consumption in a standby condition (electric power saving). FIG. 3 shows an example of the construction of a heat fixing device of this type. In the heat fixing device of this type, a nip portion N is formed by sandwiching a heat resistant belt (a fixing belt 310) between a ceramics heater 312 as a heating body and a pressure roller 330 as a pressurizing member, a recording material P, on which an unfixed toner image t to be fixed is formed and carried, is introduced between the fixing belt 310 at the nip portion and the pressure roller 330, and the recording material P is supported in a sandwiching manner and transported together with the belt 310, whereby in the nip portion, the heat from the ceramics heater 312 is given to the material P to be recorded via the belt 310 and the unfixed toner image t is hot pressed and fixed on the surface of the recording material P by this heat and the pressure load of the nip portion.
Heat resistant resins and the like are used as the material for the belt in such a belt heating type fixing device, including polyimide resins which are especially excellent in heat resistance and strength. However, in machines of high speed design and high durability design, the strength of resin films is insufficient. For this reason, the use of belts having a base layer made of metals excellent in strength, for example, stainless steel, nickel, copper and aluminum, has been proposed.
The Japanese Patent Application Laid-Open Nos. H07-114276 and 2001-006868 disclose an induction heating type in which a metal belt is used and the self-heating of this belt is caused to occur by an eddy current caused by electromagnetic induction. FIG. 4 shows an example of the construction of a heat fixing device of this heating type. FIG. 5 shows a schematic illustration of magnetic field generating means of the heat fixing device of FIG. 4. Magnetic cores 417a, 417b and 417c are members with high magnetic permeability, and an exciting coil 418 generates an alternating magnetic flux by an alternating current (a high frequency current) supplied from an exciting circuit (not shown). When this alternating magnetic flux acts on a metal layer of a fixing film, an eddy current is generated to heat the metal layer. This heat heats the fixing film via an elastic layer and a release layer of the fixing film and heats a recording material P which is fed to a nip portion N, whereby a toner image is thermally fixed. That is, there has been proposed a heat fixing apparatus in which an eddy current is generated in the belt itself or an electrically conductive member provided very close to the belt by the magnetic flux and heat is generated by the Joule heat. In a heat fixing device of this electromagnetic induction heating type, the efficiency of consumed energy can be increased because the heat generation region can be provided closer to a body to be heated.
Methods of driving a fixing belt of a heat fixing device of the belt heating type include a method in which a belt which is brought into pressure contact with a film guide which guides an inner surface of the belt and a pressure roller is driven and rotated by the rotational driving of the pressure roller (the pressure roller driving method), and a method in which conversely, a pressure roller is driven and rotated by the driving of a belt in endless belt form which is set up in a tensioned condition by a driving roller and a tension roller.
The Japanese Patent Application Laid-Open No. H07-013448 discloses as a fixing belt which is a metal belt, a fixing belt made of nickel having a thickness of 40 μm or so in which the surface roughness of a contact portion of a heater surface is less than 0.5 μm. The Japanese Patent Application Laid-Open No. H06-222695 discloses a fixing belt of nickel with a thickness of 10 to 35 μm, having a coating layer having release characteristics on an outer circumferential surface and a resin layer on an inner circumferential surface.
As described above, generally, a seamless belt base material is used in a fixing belt which is employed in an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus and the like. For example, a seamless belt base material formed from a nickel material is generally fabricated by an electroplating process (which may sometimes be called an electroforming process) which uses a nickel sulfate bath, nickel sulfamate or the like.
In this electroplating process, a mother mold having a prescribed shape is used, film formation by electroforming is performed on the outer circumference of the mother mold, and a seamless belt base material is produced by being extracted from the mother mold. However, in a conventional nickel seamless belt, the surface is oxidized when heated to not less than 180° C. during fixing. In the case of the heat fixing device of the belt heating type shown in FIG. 3, for example, the surface is scraped off due to contact with the ceramics heater 312 and the belt guide 316 and frictional resistance increases. For this reason, a torque of the fixing belt driven by the pressure roll (pressurizing member) 330 increases and it becomes impossible to obtain designed rotations.
Therefore, it has hitherto been general practice to provide a sliding layer on the belt guide side (inner surface) of the seamless belt base material. The purpose is to reduce the resistance due to contact of the fixing belt with the belt guides 316, 416a and 416b and sliding plates 340, 440 in FIGS. 3 and 4. It has been proposed to form a sliding layer by using polyimide resin. However, because the thermal conductivity of what is called resin-based materials including polyimide resin is approximately 300 times as low as the thermal conductivity of nickel, which is the base material, (nickel 0.92 W/cm·° C., polyimide resin 2.9×10−3 W/cm·° C.), the start-up time becomes long and the advantage of the nickel materials that thermal conductivity is good disappears. Polyimide resin requires high material costs, and process costs also increase because a polyimide resin film is formed on the inner surface of the belt. Furthermore, there are many cases where during the film forming process of polyimide resin, moisture is absorbed in the polyimide film and the excellent characteristics of polyimide are lost.
On the other hand, the Japanese Patent Application Laid-Open No. 2001-006868 discloses a lubricating metal layer which is such that ceramics particles or synthetic resin particles are dispersed in a metal matrix, the lubricating metal layer being formed on the surface of a heating member sliding with a support member. By providing a metal layer which is such that ceramics particles or synthetic resin particles are dispersed in a metal matrix, it is possible to reduce the sliding resistance of the surface of the heating member sliding with the support member and also to suppress an increase in the sliding resistance by an improvement of paper-feed durability. However, because the thermal conductivity is still small compared to nickel, which is the base material, this small thermal conductivity remains to be a problem to be solved in increasing the printing speed of a heat fixing device.
On the other hand, the Japanese Patent Application Laid-Open No. 2001-225134 proposes a metal tube produced by plastic forming methods. The plastic forming methods include drawing, pultrusion, processing method which involves pultruding a base material during drawing, and the like. When the thickness of a tube is to be reduced, for example, in the case of the pultrusion, it has drawbacks such that the wear of dies occurs frequently, the thickness cannot be reduced (thickness: not more than 30 μm), and the like.
In the future, requirements for energy saving and space saving will become increasingly severe, and small designs of a heat fixing device used in an image forming apparatus and small designs of the inside diameter of a fixing belt are being pursued. Therefore, a fixing belt having a metal layer is required to provide oxidation resistance at high temperatures, lubricity, thermal conductivity, thin wall designs, heat resistance, flexibility and the like.